Generally, the human body is exposed to natural radiation existing in nature. As the natural radiation existing in nature, there are: alpha rays, beta rays, and gamma rays which are radiated from radioactive isotopes present in the atmosphere, earth and rock; and cosmic rays that are radiated from space to the human body by sunlight.
In addition to natural radiation, the human body is also radiated by artificial radiation generated by humans, for example, radiation generated by products such as medical devices, TVs, fluorescent lamps, computers, electronic products, machinery, etc.
Since the radiation that is radiated to the human body has various effects thereon, the International Atomic Energy Agency has set a recommended value of 3 mSv (radiation irradiation unit) per year. Accordingly, in Korea, the radiation safety guide tolerance is set to less than 1 mSv per year.
Particularly, naturally occurring alpha radiation by radon (Rn) gas contained in the air that constitutes about 50% or more of the radiation dose to which a person is exposed. Usually, controlled radon concentration in the atmosphere varies from country to country, but it is recommended at 60 to 200 Bq/m3 or less. The Republic of Korea currently specifies radon (Rn) concentration as 4 pCi/l (148 Bq/m3) as a recommendation standard for indoor air quality.
As described above, radon (Rn) gas, which is the main cause of radiation exposure in general, continues to move to the ground via soil and gravel surrounding the building, whereafter it moves to room to room through the space of the building and through the pores of the concrete. It is known that radon (Rn) permeating from the surrounding soil is the main cause of indoor radon (Rn). In addition, building materials such as concrete, gypsum board, gravel, and brick also become sources of indoor radon Rn pollution.
In addition, since radon Rn is easily dissolved in water, it may flow indoor through movement of groundwater. Indoor movement through water occurs due to a capillary phenomena and water pressure whereby water passes through the pores of concrete. The higher the indoor temperature, the lower the pressure, and thus the more radon Rn gas flows into the indoor.
When radon Rn gas present in the atmosphere as described above is inhaled, alpha particles emitted from radon Rn are directly radiated to the lung cells, thereby destroying the cells. Thus, radon Rn is a primary carcinogen that has a very strong biological effect, such as lung cancer, on humans.
In order to accurately evaluate the indoor concentration of radon Rn, which greatly affects human health as described above, various types of measuring instruments, and various measuring methods and devices have been widely used and developed. Since radon Rn is a colorless, odorless inert gas, there is no reactivity and direct measurement is difficult. Since the frequency of alpha particles generated in an occurrence of alpha (α) decay is proportional to the radon concentration, indoor radon concentration is measured by detecting alpha particles present in the atmosphere. In other words, an accurate detection of alpha particles is a method of accurately measuring the indoor radon concentrations.
Generally, as apparatuses for detecting alpha particles, surface barrier type detectors, high purity semiconductor (pure Ge) detectors, scintillation detectors, solid state junction counters, pulsed ionization chambers, etc. are used.
Describing surface barrier type detectors, a depletion layer such as p-n junction is formed on a surface of a semiconductor due to a surface level or an oxide film, thus the vicinity of the surface becomes an obstacle of charge movements. For practical use, about 100 m/cd of gold is deposited on the surface of n-type Si, and the deposited surface is used as one electrode, and radiation is radiated to another side. Herein, there are various depletion layers having a thickness of about 50˜500 m. The depletion layer is used for detecting charged alpha particles generated due to alpha radiation since energy loss on the surface of the depletion layer is low. In addition, the depletion layer has good energy resolution. However, since it is necessary to block external light, the surface barrier type detector becomes close to a sealed type and air circulation therein is not smooth, so a forced circulation pump is mainly used. In addition, bias power having relatively high voltage must be applied since the charged particles have to collide with the surface of the semiconductor, thus the price is high and detection efficiency is low as a limitation of 2D measurement since a semiconductor detection element having a large surface area is required.
High-purity semiconductor detectors are generally as pure Ge detectors. High purity semiconductors are high-purity Ge crystals with very small impurity concentrations and defects, has a very high electrical resistance at low temperature, and are this capable a high bias voltage applied thereto. Pure Ge is different from as Ge(Li) in that the pure Ge may be stored at room temperature and is only cooled by liquid nitrogen whenever the measurement is performed. Thus, it is easy to maintain, energy resolution may be in comparison with that of Ge(Li). Accordingly, pure Ge has been put to practical use. However, the price is very expensive, and the volume is very large since liquid nitrogen cooling is essential.
Describing scintillation detectors, a phenomenon in which light is emitted by charged particles colliding with various materials has been known for a long time. However, emissions due to alpha radiation of zinc sulfide (ZnS) or a NaI coating film are particularly strong, and may be detected and counted by a magnifying glass in a dark room.
The above emission refers to a scintillation, and a substance that exhibits the above phenomenon is called a scintillator. In addition, the scintillator with a photo-multiplier is called a scintillation detector. Particularly, a method of using a coefficient as a pulse output is called a scintillation counter.
Meanwhile, a method of reading an output as a DC type is mainly used for measuring doses, and the method is called a scintillation dosimeter since the method uses a scintillator. Any one of solid, liquid, and gas may be used as a scintillator. When liquid is used, it may be called as a liquid scintillation counting device. The advantage of the scintillation detector is that the sensitivity is very high, but the disadvantage thereof is that the durability is poor since the zinc sulfide (ZnS) or the NaI coating film is weak against moisture. In addition, the price of the detector becomes expensive since an expensive photo-multiplier is essentially required.
Solid state junction counters use solid inverse-bias p-n junction semiconductors, and are counters that are configured to collect ionic charges from alpha particles passing through a depletion layer. The counters may be manufactured in a small size that is portable. However, a surface of the metal electrode of a detector thereof should not be scratched or peeled off, and no external light should be introduced. In addition, recently, such counters are mainly used to slowly measure indoor radon concentration with a low price and a simple structure by protecting a measuring sensor from light and dust by an ionization chamber (24 hours to 48 hours required only for the first data display). Solid state junction counters are not used as real-time alpha particle detection devices since the sensitivity thereof is very low as 0.02 to 0.03 CPM/pCi/l.
Pulsed ionization chambers are configured with a metal cylinder in which a probe is installed as an electrode in the center of a metal cylinder and electric field is formed thereinside by applying a bias voltage between the metal cylinder and the electrode. When alpha particles are emitted due to an occurrence of alpha decay within an ionization chamber, the alpha particles dissipate due to collision with air, but ionic charges are generated. Alpha particles may be detected by absorbing the generated ionic charges by using the probe electrode, and amplifying the absorbed charge signal. Prices of chambers are very cheap since the sensor itself is configured with a metal cylinder and a probe, and the chambers have good durability and are light-free, so ventilation may be improved. However, input impedance of the sensor is close to infinity and is sensitive to electrical noises, so it is very difficult to design a measuring circuit with a high signal-to-noise ratio. Thus, the integral type ionization chamber is widely used rather than a pulsed type due to the easy removal of noises. However, since electrical pulses have to be counted to detect alpha particles generated in the occurrence of alpha decay, a detecting circuit has to be designed in a pulsed type. Therefore, it is essential to solve the noise which is the fundamental problem of the ionization chamber.